Demands in the commercial market for increased quality in the reproduction of audio signals have led to a revival of the investigation of digital techniques which promise the possibility of preserving much of the original signal quality. A straight-forward application of conventional digital coding would lead to excessive data rates; so acceptable techniques of data compression are needed.
One such technique is proposed by M. R. Schroeder et al in "Optimizing Digital Speech Coders By Exploiting Masking Properties of the Human Ear", JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol. 66 pp. 1647-1 652 (December, 1979); and a more recent such technique is described in the article by E. F. Schroeder et al, "`MSC`: Stereo Audio Coding With CD-Quality And 256 kBit/Sec", IEEE TRANSACTIONS On CONSUMER ELECTRONICS, Vol. CE-33, No. 4, November, 1987. That article proposed the use of Transform Coding with psychoacoustic processing to reduce redundancy and to keep quantizing noise below "the signal-dependent threshold of masking". The latter requirement means that the quantization process should quantize every value to be encoded with a different precision, that is, by "bit allocation." The results achieved by the technique of that article are similar to those achievable by the use of a "tone-masking-noise" psychoacoustic threshold.